Tristimulus colour reflectance measurement of milk and

383
Lait (1992) 72, 383-391
© Elsevier/INRA
Original article
Tristimulus colour reflectance measurement
of milk and dairy products
W Kneifel, F Ulberth, E Schaffer
Agricultural
University, Department of Dairy Research and Bacteriology,
Gregor Mendel-Str 33, A-118D Vienna, Austria
(Received 20 December
1991; accepted 4 May 1992)
Summary - A tristimulus reflectance technique was applied to the objective assessment of the colour
of milk and dairy products. A variety of milk and dairy products (Iiquid milk, cultured products, cheese,
butter, milk powder) was characterized based on the L" a', b' (CIE-LAS) colour parameters. The b'
value was not suitable for estimating the B-carotene content of butter, whereas storage defects
(non-enzymatic browning reactions) of whey powder could be monitored using this parameter.
colour
1 milk product
1 reflectance
colorimetry
1 tristimulus
technique
Résumé - Méthode tristimulus
(réflexion colorimétrique)
pour la mesure de la couleur du lait
et des produits laitiers. La technique de réflexion tristimulus a été appliquée à la mesure objective de
la couleur d'un certain nombre de laits et de produits laitiers du commerce. Parmi ceux-ci, on trouve
des liquides, des produits fermentés frais tels que yaourts, diverses sortes de fromages, des beurres
d'été et d'hiver et des poudres de lait, caractérisés par leurs valeurs L', a' et b' selon la CIE. La composante jaune b' s'est révélée inadéquate à estimer la teneur en fJ-carotène du beurre, mais permet
de suivre les altérations subies par une poudre de lactosérum en cours de stockage (brunissement
non enzymatique).
couleur
/ produit
laitier / photométrie
de réflexion
INTRODUCTION
ln general, colour and shape are the major properties that give objects their individual characters. As far as foods are concerned, other sensations su ch as srnell,
taste and texturai attributes contribute to
the ove rail quality of these products. Nevertheless, in many cases food colour is
the first criterion to be perceived by the
/ méthode
tristimulus
consumer. It is weil known that the repeated recognition of a particular brand of
a Iood commodity largely depends on its
typical colour. Thus, in the food industry
the assessment of the colour of foods and
its components has become an integral
part of total quality control. Since reliablé
methodology for the objective measurement of colour has been developed, this
technique has found widespread use in
384
W Kneilel et al
many food sectors. For instance, intrumental assessment of the colour of meat and
meat products (Stolle and Paulick, 1990),
egg yolk (McCready et al, 1973) fruits and
vegetables
(Kader and Morris,
1978;
Wainwright and Hughes, 1989), sweets
and chocolate (Ugrinovits, 1987; Kneifel et
al, 1990) and coffee (Francis and Clydesdale, 1975) has been described. Several
reports concerning milk and dairy products
can also be found in the Iiterature (Bosset
et al, 1977, 1979, 1983a,b, 1986; Kammerlehner and Kessler, 1979; Desarzens
et al, 1983; Desarzens, 1988; Giangiacomo and Messina, 1988, 1989; Kneifel et al,
1992).
The purpose of this paper was to demon strate the potential inherent in objective
colour measurement and to present a survey of the colour of different milk products
as estimated by a tristimulus reflectance
technique.
MATERIALS
AND METHODS
Samp/e materia/s
A variety 01 milk and dairy products was purchased lrom local retail outlets. Whey and milk
powder samples were provided by different Austrian plants. For storage experiments, whey
powders with different water content were prepared by conditioning the products in an atmosphere 01 delined humidity provided by cabinets
containing delined salt solutions. Retail lard was
used as "relerence material" (matrix) lor colour
measurements 01 butter. 13-Carotenewas purchased from Sigma Chemicals (St Louis, USA).
pies were held at 70, SOand 90 oC for 1 and 5
min respectively in thermostated water-baths.
An oil-bath was used for heat-treatment at 100
and 120 "C using the same time conditions. Sampies were cooled in an ice-bath alter heating.
çotour
measurement
A Microcolor tristimulus colorimeter (Dr Bruno
Lange GmbH, Berlin, Germany) was used for
colour testing. Calibration was performed using
the Dr Lange "White-standard" UM 076 (standard tristimulus values: X = 69.0; Y = 73.5; Z =
77.0) as specified by the manufacturer. The
measuring principle 01 this apparatus is based
on a d/Sooptical structure, 10° standard observer, D 65 standard illuminant. A xenon flash lamp
was the light source. Each sample was tested in
4 replicates. Results were expressed using the
L*, a*, b*-system according to CIE-LAB (Commission Internationale de l'Éclairage, 1971). In
this system, L* defines the position of the sampie on the dark-light axis, e: on the green-red
axis, and b* on the blue-yellow axis.
Liquid and semi-solid products were filied to
the engraved mark of a "Iiquid sample" quartz
cuvette, and subsequently covered with a PTFE
piston. Care was taken not to include air bubbles in the liquid. Fruit yogurts were stirred with
a spoon and subsequently poured through a 1mm sieve to remove larger partieles before
measurement. Powdered products were transferred into the "powder sample" quartz cuvette.
The filled cuvette was then tapped slightly on a
solid support in order to ensure a homogeneous
sample distribution. Both types of cuvette were
placed on the head of the Microcolor measuring
unit and covered with a Iid before starting the
measuring cycle. In the case of solid samples
Iike cheese, the measuring unit was placed directly onto the specimen which had been freshly
eut from the cheese sample. Ali samples were
measured at 20 ± 1 "C alter an equilibration
time of at least 1 h (Burton, 1956).
Heat treatments of milk
Screw-capped glass tubes containing 20-ml portions of raw milk (4.1% fat) were immersed in a
boiling water-bath until the required heating
temperature was reached. Therealter, the sam-
Other physica/
and chemica/ parameters
Dry matter of powdered products was determined according to FIL-IDF standard (Interna-
Col our measurement
tional Dairy Federation, 1964). Sieve fractions of
milk powder were collected as described by
Haugaard-Sorensen
et al (1978). Total hydroxymethylfurfural
(HMF) concentration
was determined according
to the spectrophotometric
method of Keeney and Bassette (1959). The extent of homogenization
was estimated as outIined by Schneider and Roeder (1979). Carotene content of butter fat was determined
according to Pardun (1969). Melted butter oil
was decolorized
with charcoal, following the
methodology proposed by Schaap and Rutten
(1974).
RESUL TS AND DISCUSSION
Precision
of cotour measurement
The precision of the colour reflectance
method was determined
by repeatedly
measuring pasteurized who le milk (3.6%
fat). The within-run relative standard deviation (RSD) (N = 10) was 0.06% for the L*
value, 2.99% for b*, and 0.65% for a*. The
between-run RSD (N = 10) was 1.21 %,
3.11 % and 1.44%, respectively.
cotour
parameters
of milk and dairy products
A characterization of the col our of different
dairy products is given in table 1. As can be
seen from these data, Iiquid and cultured
milk products tested had slight 'green' and
'yellow' components. The values found for
pasteurized milk are partly different from
those reported by Giangiacomo and Messina (1988) (L* = 88.2, a* = -4.35, b* = 5.40)
and by Bosset and Blanc (1978) (L = 95.5,
a = -2.0, b = 12.6). The observed differences between our results and those reported by Bosset and Blanc (1978) were
obviously due to the fact that they used the
Hunter-L,a,b system. Compared to retail
Iiquid milk (3.6% fat, homogenized), the b*
of dairy products
385
value of set-style yogurt increased by 1.3
units. As demonstrated
by Giangiacomo
and Messina (1989), this difference is
caused by the acidification and coagulation
process. Most of the cheese types can be
characterized as exhibiting slight 'red' and
pronounced 'yellow' colour components.
The L* values of the liquid and cultured
milks indicated a high degree of whiteness
and gave a rather consistent pattern, ranging from 81.7 to 87.5. Skimmed products
tended to be lower in L* than their corresponding full-fat products (producing
a
higher degree of light scattering). The colour parameters
of fresh and feta-type
cheese closely resembled those data obtained with Iiquid products. On the other
hand, ripened cheese varieties showed a
marked variation in colour values. lt has
been shown previously
(Bosset et al,
1977) that several parameters, eg texture
(holes and cracks), surface properties, oil
exudation, sam pie thickness and slicing
technique can influence the results of colour measurements
on cheese samples.
Mainly due to the typical colour of the additives, the colour parameters of fruit yogurts
varied as expected to a great extent.
The colour parameters of full-cream milk
powders
differed
from those
of the
skimmed milk powders (table 1). The relative magnitude of this difference was mostIy pronounced with respect to the b* value.
However, it should be taken into consideration that in the CIE-LAB system the Y"
parameter is used for the computation of
L*, a* and tr, meaning that ail parameters
are interrelated. Obviously, the powder colour is influenced by the fat content via the
liposoluble ~-carotene. The colour of powdered milk products may also be influenced by technological parameters and by
the geographical as weil as c1imatic conditions of milk production (table 1). lt is further evident from the colour data given in
figure 1 that there were no marked differences in the L* and a* values of the sieve
386
W Kneifel et al
Table 1. Typical colour parameters of different milk products (mean values of at least 3 different replicate samples); C: dark (0), light (100); a*: green (-), red (+); b": blue (-), yellow (+).
Composantes typiques de la couleur de produits laitiers différents (valeur moyenne d'au moins 3
échantillons mesurés en triple); L *; (foncé) (0), clair (100); a * vert (-), rouge t-): b * bleu (-), jaune (+).
Product type
Pasteurized
Fat content (%)
milk
UHTmilk
Cream
Coffee cream
Cultured buttermilk
Cultured milk
Yogurt (set-style)
Yogurt with fruits
apricot
strawberry
blueberry
raspberry
Yogurt dessert product
vanilla
coffee
Fresh soft cheese
Gervais
Processed cheese
Camembert (surface)
(interior)
(surface)
(interior)
Brie (surface)
(interior)
Feta cheese
Roquefort
Tilsit chee se
Tilsit Swiss type
Edam cheese
Gouda cheese
Swiss type cheese
Appenzell type cheese
Full-cream milk powder
Skimmed milk powder
(Austrian origin)
(American origin)
(New Zealand origin)
L*
a*
b*
3.6
4.5
2.5
36.0
10.0
0.1
3.6
1.0
3.6
81.7
86.1
86.2
86.0
88.1
86.9
86.5
87.5
85.9
86.6
-4.8
-2.1
-1:7
-2.0
-0.2
-0.5
-2.6
-1.5
-2.5
-1.9
4.1
7.8
7.5
7.9
8.8
8.6
6.9
6.5
8.8
9.1
3.2
3.2
3.2
3.2
82.3
77.0
52.9
67.8
1.3
9.1
20.6
13.1
10.7
4.9
-7.3
2.4
7.0
7.0
< 1.0
10.0
20.0
40.0
65.0
55.0
45.0
45.0
50.0
35.0
45.0
25.0
45.0
30.0
45.0
45.0
25.0
83.7
66.5
85.7
86.1
85.6
85.0
85.9
91.0
95.7
85.6
94.6
86.8
95.9
90.2
93.5
92.8
77.2
79.9
72.9
79.8
82.6
72.7
71.9
95.6
0.9
5.3
-0.9
-0.9
-0.3
1.3
1.1
3.0
0.1
3.3
0.5
3.2
0.2
2.7
-1.1
-1.4
3.1
3.1
4.0
4.2
3.6
0.6
2.2
-3.6
11.8
18.7
10.4
10.4
10.6
10.7
12.0
18.6
5.2
26.9
5.9
27.7
4.1
26.5
11.0
14.5
28.8
24.1
27.6
32.2
27.1
20.9
25.5
19.8
1.0
1.0
1.0
94.9
92.5
93.0
-1.7
-2.6
-2.2
11.3
18.3
16.4
< 0.1
60.0
60.0
Colour measurement of dairy products
Size distribution %
60 .------'---'-=-----------,
90
cotour of butter
150
200
96,3
-3,3
18
96,5
-3,6
19,8
Mesh
L-
ab·
93,6
-3,7
20,8
95,3
-3,5
20,1
95,8
-3,5
19,3
Fig 1. Particle size distribution and colour parameters L*, a* and b* of full-cream milk powder.
Distribution
des particules
387
et des composantes
L *, a * et b * de la couleur de la poudre de lait entier.
fractions collected from full-cream milk
powder. Only the b* values differed to a
certain extent. This observation is also in
agreement with the findings of Bosset et al
(1979).
Results of colour measurements on butter
samples are listed in table II. Colour differences between summer and winter butter
were apparent and mainly due to differing
~-carotene contents. Moreover, it is evident from the data that the L*, a*, b* parameters were strongly influenced by the
sample temperature. This effect is obviousIy caused by the temperature-dependent
extent of fat crystallization (solid/liquid ratio). In the case of colour measurements
on butter, it is therefore particularly necessary to perform the tests under defined
temperature-time
conditions to obtain a
stable crystal modification.
ln another series of experiments, an attempt was made to estimate
the ~carotene content of butter based on the b*
values as an indicator for the yellow colour. Different amounts of ~-carotene were
added to a decolorized butter oil, and for
reference purposes also to lard, which is
known to be completely colorless. The b*
values measured are graphically presented in figure 2. Correlation coefficients (regression lines) calculated were 0.95 (y =
3.020 + 2.010x; N = 6) for the butter oil,
and 0.98 (y = 2.626 + 13.126x; N = 5) for
Table II. Colour parameters of butter samples at different temperatures.
Composantes de la couleur des échantillons de beurre mesurés à différentes températures.
Butter type
Sample temperature
(%)
L*
a*
b*
Summer butter
10
16
18
22
63.2
61.8
59.9
57.1
3.9
3.0
2.5
2.5
31.1
30.6
30.5
30.4
Winter butter
10
16
18
22
70.8
65.6
63.8
61.3
4.7
3.7
3.4
2.8
28.3
29.8
29.7
29.6
388
W Kneifel et al
b·value
40.-------------~-__,
30
20
•
o
246
Butterail
Lard
8
D-carotene
la
12
U9/9
Fig 2. Relationship between l3-carotene content
and b* values of butter oil and lard, after addition of known amounts of l3-carotene.
Rapport entre la teneur en {3-carotène et les valeurs b * de la matière grasse liquide du beurre
et du saindoux enrichis en {3-carotène.
trates to high tempe ratures has been weil
documented
(eg Horak and Kessler,
1981). Using laboratory experiments, we
were not able to detect significant changes
in the colour of non-homogenized
milk under time-temperature
conditions
usually
applied for milk pasteurization within the
range of 70-90 oC (table III). Pronounced
changes could be reqistered only on severe heat treatment of milk in conditions
resembling autoclaving or UHT treatment.
However, Bosset et al (1979) indicated a
sm ail but significant increase of the Hunter-L,a,b components with the temperature
applied to homogenized milk.
It has been shown that non-enzymatic
browning reactions also occur during prolonged storage of dried milk products
Table III. Colour parameters of Iiquid milk heated at different time-temperature conditions.
the lard samples. Although a close corre la'tion was observed between B-carotene
concentrations and b* values, an accu rate
estimation of the 13-carotene content was
not possible based on b* value measurement. The deviations of the chemically determined 13-carotene content from the results obtained by colour measurements
may be due to the varying crystal structure
of the products as weil as to the dependence of b* on other colour parameters (eg
L*). A similar divergence is evident from the
results reported by Desarzens et al (1983)
who were unable to find constant relationships between the vitamin B2 contents and
the L,a,b parameters of milk samples exposed to light for different time periods.
Changes of cotour during processing
and storage of milk products
The formation of coloured products in the
course of heating of milk or milk concen-
Composantes de la couleur des laits chauffés a
température et à durée contrôlées.
L*
a*
b*
umol HMF/I
70 -c
1 min
5 min
79.4
79.6
-7.5
-7.6
4.8
5.6
3.2
3.1
80 -c
1 min
5 min
79.0
79.5
-7.2
-7.4
4.7
5.6
3.1
3.3
90 -c
1 min
5 min
78.9
80.1
-7.4
-7.5
4.3
5.7
3.0
3.5
100 -c
1 min
5 min
79.9
80.7
-7.4
-7.3
5.0
5.6
3.2
5.2
120 -c
1 min
5min
20 min
80.3
81.5
81.5
-7.4
-5.8
-5.4
5.1
6.9
7.2
4.3
17.1
58.8
Heating
conditions
Colour measurement
389
of dairy products
40°C
HMF ",mol/100g
60
.L.........
-----~-
40
20
5
.
o
.
,
,--------...,.60
20,-----------,
15
~:::::::==~=444O
3.00/010,!~:::~====~===1
20
'----'--_--'-_-'-_----'0
20
15
5.0°/0 ;.-.,..,.
10
20
5
0
o
.
20
40
60
(Renner, 1988; Kneifel, 1989). Hitherto,
the HMF value has mainly been used to
describe these alterations. To demonstrate
the relationship between powder coloration
and HMF content, whey powder samples
were stored at different temperatures (20,
30, 40 oC) and sampled periodically (fig 3).
As the water content of the product influences the extent and the velocity of Maillard reactions, this parameter was adjusted
to 1.5, 3.5 and 5.0% (w/w). The b* value
proved to be the most suitable indicator for
the detection of changes in colour. As can
be seen from these graphs, browning reactions of whey powders were pronounced at
high temperatures and water contents, respectively. For example, the HMF values
of samples with a moisture of 1.5, 3.5 and
20
40
5.0% which were stored at 30 "C increased to 148%, 160%, and 162% of their
initial values. By contrast, the corresponding b* values increased to 108%, 124%
and 132%. Although the HMF value was
generally more sensitive in detecting these
alterations, reflectance colorimetry was a
more rapid and simple means for the assessment of storage defects in whey powder.
CONCLUSIONS
Tristimulus colour reflectance measure. ment is a tool which can be utilized to obtain additional objective and well-defined
physical data on milk and dairy products.
390
W Kneifel et al
This technique not only yields basic information on the colour of milk and dairy
products, but also enables a precise control of the food quality du ring manufacturing of selected dairy foods or during prod.uct development. Depending on the local
legal situation, the colour values can be
used as a basis for carrying out corrections of a product's colour (eg by addition
of permitted food dyes or 13-carotene).
Particularly
and dessert
ment may be
ing the desired
in the case of fruit yogurts
products, colour measureof assistance in standardizcolour intensity.
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